O2 sensor

[raelph]

My 2003 avensis verso wagon is shuddering while idling.

It has trouble gaining speed and shudders intermittently while driving. 

Its probably a fuel problem, but mechanic wants $2000+ to change o2 sensors.

So i will do myself 

Possibly that will alleviate the problem, but more possibly not. However, sensors may help determine problem .

My question is ..there are 4 sensors ..two above cylinder 2 and 3 , i'm led to believe...and two below ( 1 and 4)

Internet suggests post CAT and Pre Cat sensors.

I assume upper sensors are pre cat.

But post Cat? Below. ? Is that correct? And just where is the Catalytic converter located? I thought it was just before muffler. But no sensors under car that i can see. Any help with knowledge is appreciated. Expensive even to install myself . Car is only worth $4000 max. So spending $2000 for mechanic only to find more problem ( fuel filter?) Really hurts

[raelph]

OK I have changed the coils and spark plugs  and the Shudder is GONE....

But when I put foot to gas I get a NO explosion.

And car does not speed up. Then it starts again when gas is applied lightly.

Possibly the Fuel Filter. (o2 sensors still not replaced)

But I can't find anywhere where it shows how to replace fuel filter DIY. Any help would be appreciated.

thanks

Raelph